High speed downlink packet access (HSDPA) supports the introduction of high bit rate data services in WCDMA (wideband code division multiple access) based communication systems which will increase network capacity while minimizing operator's investment. It provides a smooth evolutionary path for universal mobile telecommunication system (UMTS) networks to higher data rates and higher capacities. The introduction of shared channels for different users offers the possibility to use channel resources more efficiently in the packet domain, which is at the same time less expensive for users than dedicated channels.
The higher data rate will be achieved by using a higher modulation technique (16-QAM), new methods of error correction and a highly dynamic allocation of physical resources through the MAC (medium access control) layer. This modified MAC layer is called MAC-hs and is implemented in the base station, which in the UMTS case is also referred to as a node B.
The new layer MAC-hs needs to know the available channelisation codes which are also called WCDMA spreading codes. According to a standard established by the third generation partnership project (3GPP), 3GPP Release 5, the channelisation codes available for the HS-DSCH (high speed downlink shared transport channel) packet scheduling in a radio cell are explicitly signalled by the RNC (radio network controller) to the node B. It is assumed that the node B transmits HS-DSCH using exactly these channelisation codes signalled by the RNC for the HSDPA packet scheduling in a cell, and no other channelisation codes. This higher signalling is defined in the 3GPP document 3GPP TS 25.433 NBAP. It should be noted that the set of channelisation Codes available for HS-DSCH scheduling in a cell is not known to the user equipments (UEs) in the cell.